UNIVERSITY    OF    CALIFORNIA    PUBLICATIONS 

IN 

AGRICULTURAL    SCIENCES 

Vol.  1,  No.  4,  pp.  51-62  May  15,  1913 


THE  ALUMINUM  REDUCTION  METHOD  AS 
APPLIED  TO  THE  DETERMINATION  OF 
NITRATES  IN  "ALKALI"  SOILS 

BY 

PAUL  S.  BURGESS 


While  the  phenoldisulphonic  acid  method  for  determining 
nitrates  in  soils  at  present  offers  the  most  speedy  and  satisfactory 
means  of  ascertaining  the  nitrate  content  of  soils  free  from 
"alkali,"  it  has  been  shown1  to  be  of  questionable  value  when 
employed  with  soils  containing  even  small  amounts  of  soluble 
salts,  and  especially  in  the  presence  of  the  chlorides  and  sulfates 
of  the  alkalies.  Since  this  is  true,  and  further  since  the  number 
of  nitrate  determinations  on  soils  containing  "alkali"  is  con- 
stantly increasing,  due  to  the  great  increase  in  our  research  work 
on  "alkali"  problems,  both  chemical  and  bacteriological,  it  was 
deemed  by  the  writer  to  be  a  matter  of  importance  to  establish 
a  method  for  the  determination  of  nitrates  which  was  not  affected 
by  the  presence  of  soluble  salts. 

The  possible  methods  considered  were  Busch's  "nitron"  pro- 
cess, the  methods  depending  upon  the  liberation  and  subsequent 
measuring  of  nitric  oxide,  and  the  reduction  methods  in  which 
the  nitrate  nitrogen  is  reduced  to  ammonia  and  either  titrated 
against  a  standard  acid  solution  or  Nesslerized. 

Busch's  "nitron"  process2  was  rejected  for  the  following 
reasons.     A  number  of  the  acids  (both  organic  and  inorganic). 


lUniv.  of  Calif.  Publ.  Agr.  Sci.,  vol.  1,  no.  2,  pp.  21-37.     Utah  Agr. 
Exp.  Sta.  Bull.,  106. 

2Ber.  Dent.  Chem.  Gsell.  38  (1905),  3,  pp.  861-866. 


52  University  of  California  Publicatiom  in  Agricultural  Sciences    [Vol.  1 

and  salts  found  in  soils,  form  insoluble  compounds  with  the 
"nitron"  (1.4-diphenyl-3.5  endanilodihydrotryazol)  as  well  as 
does  the  nitrate  radicle.  The  sterilization  of  soils  also  appears 
to  liberate  substances  which  interfere  with  the  crystallization  of 
the  "nitron"  nitrate.3  The  occurrence  of  soluble  organic  matter 
invariably  present  in  the  soil  solutions  tends,  in  many  cases,  to 
vitiate  completely  the  results.4  The  calcium  oxid,  which  we  use 
to  coagulate  the  clay  before  filtration,  comes  down  as  the  car- 
bonate in  the  filtrate,  thus  making  a  gravimetric  determination 
impracticable.  (Lime  has  been  shown  to  effect  the  least  loss  of 
nitrates  of  any  of  the  common  coagulants.)  The  trouble  and 
cost  of  procuring  the  reagent  also  militated  against  the  use  of 
this  method.  The  Schulze-Tiemann,5  Schlosing- Wagner,6  and 
similar  methods  which  depend  upon  the  liberation  and  subse- 
quent measurement  of  the  nitric  oxide  from  the  nitrates  did  not 
appear  feasible  because  of  the  errors  introduced  through  atmos- 
pheric conditions,  the  expense  of  apparatus  for  a  large  number 
of  determinations,  and  the  length  of  time  necessary  for  the 
operations  involved. 

Thus  all  but  the  reduction  methods  were  eliminated.  In 
1890  the  Agricultural  Experiment  Station7  at  Halle,  Germany, 
perfected  a  reduction  method  for  the  determination  of  nitrogen 
in  nitrates.  They  used  zinc  dust,  iron  filings,  and  a  solution 
(sp.  gr.  1.3)  of  sodium  hydrate.  The  presence  of  chlorides  and 
sulfates  did  not  impair  the  accuracy  of  the  determination.  Sev- 
eral modifications  of  this  reduction  process  are  now  used.  In 
the  modified  Ulsch8  method  sulfuric  acid  and  reduced  iron  are 
employed  to  liberate  the  nascent  hydrogen,  an  excess  of  mag- 
nesium oxid  being  added  just  before  distillation.  In  the  Devarda0 
method  an  alkali,  an  alloy  of  aluminum,  copper  and  zinc,  and 
ethyl  alcohol  are  all  used  to  effect  the  reduction.     M.  E.  Pozzi- 


3  J.  Litzendorff,  Ztschr.  Angew.  Chem.  20  (1907),  51,  pp.  2209-13. 

4  Mich.  Exp.  Sta.  Rep.  for  1911,  pp.  178-181. 

■>  I :< .Inn.   Ztschr.  Zuckerind.   25    (1900),  p.  356,  abs.   in  Chem.   Centrbl. 
(1901),  I,  22,  p.  1216. 

sKonig's  CJntersuch.  Landw.  Stoffe,  p.  151. 
"  Experiment  Station  Record,  vol.  V,  pp.  464-465. 
b  New  Jersey  Exp.  Sta.  Rep.  for  1892,  pp.  188-193. 
0  Analyst,  35  (1910),  412,  p.  307. 


1913]  Burgess:   Reduction  Methods  for  Soil  Nitrates  53 

Escot10  utilizes  aluminum  filings,  mercuric  chloride  and  a  solution 
of  potassium  hydrate.  J.  T.  Bornwater11  uses  aluminum  filings 
and  a  solution  of  potassium  hydrate. 

Still  further  elimination  was  thus  necessary  among  the  reduc- 
tion methods.  The  modified  Ulsch  method  presented  certain 
difficulties  of  technique  which,  in  the  case  of  numerous  deter- 
minations, would  render  the  method  impracticable.  The  alumi- 
num reduction  method  has  been  heretofore  used  successfully  in 
water  analysis,12  and  it  appeared  advisable  to  make  a  study  of 
it  as  applied  to  determinations  of  large  amounts  of  nitrates  as 
met  with  in  soil  work;  it  further  seemed  important  to  determine 
the  feasibility  of  its  use  for  a  large  number  of  determinations 
made  simultaneously.  Such  other  factors  also  as  the  amounts 
of  aluminum  and  of  alkali  to  employ,  the  length  of  time,  and  the 
temperature  for  reduction,  demanded  a  careful  test. 

Description  of  the  Method 

One  hundred  grams  of  the  soil  in  which  the  nitrates  are  to 
be  determined  are  placed  in  round-bottomed,  enameled  cereal 
dishes  which  have  a  capacity  of  about  800  c.c.  Mortars  were 
used  at  first,  but  the  cereal  dishes  were  found  to  be  much  lighter, 
easier  to  manipulate,  and  less  expensive.  Two  grams  of  pow- 
dered CaO  and  exactly  200  c.c.  of  distilled  water  are  added  to 
each  dish.  The  contents  of  each  dish  are  now  thoroughly  ground 
and  mixed  with  a  pestle  for  from  3  to  5  minutes,  after  which 
the  soil  and  clay  are  allowed  to  settle  for  15  or  20  minutes,  and 
are  then  filtered  through  paper.  It  may  be  said  here  that  the 
solutions  should  never  be  allowed  to  stand  over  2  to  3  hours,  as 
there  occurs  a  noticeable  loss  of  nitrates,  possibly  due  to  dentri- 
fication.  In  case  it  is  impossible  to  proceed  at  once  with  the 
determination  a  few  drops  of  chloroform13  may  be  added.  One 
hundred  c.c.  portions  of  the  filtrates,  obtained  as  above  described, 
are  placed  in  400  c.c.  casseroles  and  2  c.c.  of  a  50  per  cent  XaOH 
solution,  free  from  nitrates,  added  to  each.    These  are  then  boiled 


io  Ann.  Chim.  Analyt.  14  (1909),  12.  pp.  445-446. 

ii  Chem.  Centbl.  (1906),  I,  8,  p.  703. 

i2Amer.  Jour.  Pub.  Hyg.,  vol.  XIX,  3,  p.  1. 

i3V.  I.  Sazanov,  Abs.  in  Centbl.  Zuckerindus.  15  (1907).  34,  p.  923. 


54 


University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


down  to  about  half  their  original  volume  to  drive  off  ammonia, 
the  residues  washed  into  125  c.c.  Jena  test  tubes,  diluted  to 
100  c.c.,  and  a  strip  of  aluminum  (about  150  X  6  X  -4  mm., 
weighing  approximately  one  gram)  added  to  each.  The  tubes 
are  then  stoppered  with  one-hole  rubber  stoppers  carrying  bent 
glass  tubes,  each  of  which  has  been  drawn  out  to  a  fine  capillary 
tip.  The  solutions  are  allowed  to  remain  in  the  tubes  from  11 
to  14  hours  at  a  constant  temperature  of  from  20°  to  22°  C. 
(about  our  laboratory  temperature).  In  case  of  large  amounts 
of  nitrogen  the  temperature  is  of  prime  importance,  a  lower 
temperature  giving  incomplete  reduction  in  the  above  mentioned 
time,  while  a  higher  temperature  may  induce  a  considerable  loss 
of  ammonia.  After  reduction  the  contents  of  the  test  tubes  are 
washed  into  distilling  flasks,  about  300  c.c.  of  distilled,  ammonia- 
free  water  is  added,  and  the  ammonia  distilled  off  and  caught  in 
N/10  HOT,  the  excess  of  acid  being  titrated  against  N/10  NH4OH. 

We  run  24  or  36  determinations  simultaneously,  avoiding  a 
loss  of  time  by  allowing  the  reduction  to  take  place  over  night, 
either  in  an  incubator  kept  at  20°-22°  C,  or  when  temperature 
conditions  are  right,  in  the  laboratory. 

The  cut  below  shown  is  a  photograph  of  a  rack  which  the 
writer  had  made  for  the  purpose  of  holding  36  of  the  large  test 
tubes  while  reduction  was  taking  place.  The  holes  are  all  num- 
bered, thus  doing  awray  with  the  necessity  of  marking  the  glass 
tubes,  the  determinations  being  run  in  rotation. 


1913]  Burgess:   Reduction  Methods  for  Soil  Nitrates  55 

Test  of  the  Reduction  Method  With  Large  Quantities  of 

Nitrate 

In  soil  work  considerable  amounts  of  nitrates  are  often 
encountered.  The  first  series  of  experiments  were  thus  made 
to  determine  whether  or  not  the  reduction  method  would  prove 
accurate  in  the  presence  of  from  30  to  60  mgs.  of  nitrate  nitrogen. 

The  results  set  forth  in  the  following  table  are  averages  of 
several  analyses  made  at  the  same  time  and  under  similar  condi- 
tions. A  comparison  is  also  made  here  of  the  reduction  method 
with  the  phenoldisulphonic  acid  method. 

TABLE  I 

A  Comparison  of  the  Eeduction  Method  and  the  Phenoldisulphonic 
Acid  Method  With  Large  Amounts  of  Nitrate  Nitrogen 


No. 

Nitrate  N 
added 
Mgs. 

Reduction 

Method 

Nitrate  N 

recovered 

Mgs. 

Phenoldisulphonic 
Acid  Method 

Nitrate  N 

recovered 

Mgs. 

1 

100 

97.86 

97.00 

2 

50 

49.00 

48.25 

3 

25 

24.22 

25.40 

We  thus  see  that  the  reduction  method,  even  where  no  salts 
are  present,  but  where  large  amounts  of  nitrates  are  found,  is 
slightly  more  accurate  than  the  phenoldisulphonic  acid  method. 

The  Effect  of  "Alkali"  Salts  on  the  Reduction  Method 

Method 


» i 


As  stated  above,  in  our  research  work  on  different  "alkali 
problems  in  soils,  more  especially  in  soil  bacteriology,  and  plant 
physiology,  considerable  amounts  of  salts  are  often  used  in  soils 
in  which  later  the  nitrate  content  must  be  ascertained.  Besides 
it  is  frequently  necessary  to  determine  nitrates  in  soils  of  the 
arid  regions  which  contain  considerable  quantities  of  "alkali." 
It  is  important  therefore  to  ascertain  if  the  method  herein  pro- 
posed is  in  any  wise  affected  by  salts.  Therefore  the  following 
tests  were  carried  out.     The  salts  employed  were  "Baker's  An- 


56 


University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


alyzed  Chemicals."  All  the  reagents  upon  analysis  were  found 
to  be  free  from  nitrogen.  The  salts  were  added  in  solution  from 
accurately  graduated  pipettes  and  burettes.  The  reduction 
period  for  all  the  samples  was  11  hours  at  a  temperature  of 
20°-22°  C. 

The  following  table  shows  the  results  obtained  in  a  series 
of  nitrate  determinations  made  by  the  reduction  method  in  the 
presence  of  "alkali."  The  details  with  reference  to  these  deter- 
minations are  also  given  in  the  table,  and  the  figures  are  averages 
of  closely  agreeing  duplicate  or  triplicate  analyses.  This  state- 
ment applies  also  to  the  following  tables. 


TABLE  II 

Effects 

of  NaCl 

NaCl 
added 
Grams 

Nitrate  N 
added 

Mgs. 

Aluminum 
Reduction 

Method 

Nitrate  N 

recovered 

Mgs. 

Phenoldisulphonic 

Acid  Method 

Nitrate  N 

recovered 

Mgs. 

0.2 

100 

97.90 

79.50 

0.2 

'  50 

49.10  . 

43.00 

0.2 

25 

24.50 

20.75 

Effects  of  Na,S04 


Na2S04 
added 
Grams 

Nitrate  N 
added 

Mgs. 

Aluminum 
Reduction 

Method 

Nitrate  N 

recovered 

Mgs. 

Phenoldisulphonic 

Acid  Method 

Nitrate  N 

recovered 

Mgs. 

0.35 

100 

98.42 

72.75 

0.35 

50 

49.42 

38.20 

0.35 

25 

24.36 

19.75 

Effects  of  NAoC03 


Na„CO, 

added 

Grams 

Nitrate  N 
added 

Mgs. 

Aluminum 
Reduction 

Method 

Nitrate  N 

recovered 

Mgs. 

Phenoldisulphonic 

Acid  Method 

Nitrate  N 

recovered 

Mgs. 

0.1 

100 

98.14 

86.25 

0.1 

50 

49.42 

42.50 

0.1 

25 

24.50 

25.20 

1913]  Burgess:   Seduction  Methods  for  Soil  Nitrates  57 

TABLE  II—  (Continued) 

Effects  of  "Mixed  Alkali"  Salts 


Salts 

added 

Grams 

0.2  NaCl        ] 

Nitrate  N 
added 
Mgs. 

Reduction 

Method 

Nitrate  N 

recovered 

Mgs. 

Acid  Method 

Nitrate  X 

recovered 

Mgs. 

Phenoldisulphonic 

0.35  Na,S04  }. 

100 

99.54 

65.00 

0.1  Na2C03    J 

0.2  NaCl        ] 

0.35  Na2S04  j> 

50 

50.05 

33.00 

0.1  Na2C03    j 

0.2  NaCl        1 
0.35  Na:S04  [. 
0.1  Na2C03    J 

25 

24.57 

12.00 

That  the  salts  present  have  little  effect  on  the  accuracy  of 
the  determination  by  the  reduction  method,  and  the  superiority 
of  the  latter  over  the  phenoldisulphonic  acid  method,  is  clearly 
shown  by  a  comparison  of  the  last  two  columns  in  the  foregoing 
table.  Where  large  losses  of  nitrates  are  induced  by  the  presence 
of  NaCl  and  Na2S04  with  the  second  named  method,  there  are 
only  slight  losses  when  the  reduction  method  is  employed,  and 
these  are  found  by  a  comparison  with  Table  I  to  be  apparently 
due  to  other  causes. 

In  most  cases  where  the  reduction  method  is  used  there  are 
slight  losses  of  ammonia,  although  in  a  few  individual  analyses 
all  of  the  nitrogen  was  recovered.  It  may  be  stated  here  that 
several  analyses  were  run  on  solutions  containing  100  mgs.  of 
nitrate  nitrogen,  placing  rubber  stoppers  in  the  large  reducing 
test  tubes,  carrying  bent  glass  tubes  as  traps  and  connecting  with 
test  tubes  containing  10  c.c.  of  N/10  HC1  each.  These  were  later 
titrated  against  N/10  NH4OH  and  the  results  added  to  the  figures 
found  by  distillation.  By  using  this  extra  process  we  were 
able  to  recover  all  the  N  as  NH3,  but  it  was  not  found  to  be 
necessary  where  the  smaller  amounts  of  nitrate  were  present. 
It  should  be  remembered  that  in  soil  work  over  30  mgs.  of  nitrate 
nitrogen  per  100  grams  of  soil  are  infrequently  found,  and  of 
this  amount  only  an  aliquot  (one-half)  is  taken  for  the  actual 
analysis.  In  nitrification  work  also,  especially  where  the  pro- 
duction of  nitrates  is  intense,  the  difference  between  parallels  is 


58  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

often  from  one  to  one  and  one-half  milligrams  of  nitrogen,  or 
about  the  amounts  which  appear  to  be  lost  where  large  quantities 
of  nitrates  are  determined  by  this  method. 

As  a  means  of  comparison,  the  same  amounts  of  "alkali" 
salts  and  nitrate  were  added  and  the  entire  series  analyzed  by 
the  phenoldisulphonic  acid  method.  The  results  of  this  work  are 
shown  in  the  last  column  of  Table  II.  The  standard  method 
here  calls  for  the  use  of  2  c.c.  of  the  phenoldisulphonic  acid,  but 
we  found  that  even  where  no  foreign  salts  were  present,  if  the 
amount  of  nitrate  nitrogen  exceeded  about  10  mgs.  4  c.c.  were 
necessary  to  complete  the  reaction. 

The  results  of  tests  of  the  colorimetric  method  where  large 
amounts  of  nitrates  were  present,  using  respectively  2  and  4  c.c. 
of  the  phenoldisulphonic  acid,  follow: 


TABLE 

III 

Nitrate  N 
recovered 

Nitrate  N 
recovered 

To. 

"Alkali" 

salts 
present 

Nitrate  N 
added 

Mgs. 

using  2  c.c. 
phenoldisul- 
phonic acid 
Mgs. 

using  4  c.c. 
phenoldisul- 
phonic acid 
Mgs. 

1 

0 

100 

58.00 

97.00 

2 

0 

50 

35.00 

48.25 

3 

0 

25 

23.60 

25.40 

To  ascertain  whether  or  not  4  c.c.  portions  of  the  acid  were 
sufficient,  6  c.c.  quantities  were  tried.  No  gains  in  the  amount 
of  nitrate  nitrogen  recovered  here  resulted.  Four  c.c.  portions 
were  employed  in  the  experiments  reported  in  Table  II. 

It  is  interesting  to  note  that  while  my  results  confirm  the 
work  of  Lipman  and  Sharp  on  the  effects  of  NaCl  and  Na2S04 
on  the  phenoldisulphonic  acid  method,  they  are  partly  at  variance 
with  them  on  the  effects  of  Na2COo.  The  investigators  named 
found  that  the  nitrate  determination  by  the  method  mentioned 
was  in  no  wise  affected  by  Na.,C03,  but  it  should  be  recalled  that 
they  employed  comparatively  small  quantities  of  nitrates  and 
carbonates  and  that  in  the  presence  of  larger  quantities  the 
chances  of  error  are  magnified. 


1913]  Burgess:   Reduction  Methods  for  Soil  Nitrates  59 

The  Effect  of  Soluble  Organic  Materials  on  the  Reduction 

Method 

After  having  found  that  the  soluble  mineral  salts  had  no 
effect  on  the  accuracy  of  the  reduction  method,  it  was  deemed 
advisable  to  ascertain  whether  or  not  the  organic  matter,  which 
is  always  present  in  the  soils  to  a  greater  or  less  extent,  interferes 
with  it.  Such  organic  materials  may  be  roughly  divided  into 
two  great  classes :  the  humates,  or  the  salts  of  humic  acid  with 
the  alkalies  of  the  soil,  and  the  soluble  carbohydrate  material.  By 
simply  triturating  the  soil  sample  with  pure  water  but  a  very 
small  percentage  of  the  former  class  of  compounds  is  ever  ex- 
tracted, while  soluble  carbohydrates  are  present  in  soils  onty  in 
exceedingly  small  quantities,  except  in  rare  cases.  In  these  tests 
dried,  water  solube  humus  and  dextrose  were  used  and  two  sets 
of  experiments  were  run,  in  one  of  which  the  solution  to  be 
reduced  contained  .2  per  cent  of  dried  humus  and  the  other  of 
which  contained  1  per  cent  of  dextrose.  The  addition  of  humus 
produced  a  very  dark  brown  solution,  much  darker  in  fact  than 
any  solution  that  can  be  obtained  by  triturating,  even  soil  high 
in  humus,  with  water.  The  analytical  procedure  was  the  same 
as  that  above  given  and  the  results  are  expressed  in  Table  IV, 
and  represent  averages  of  closely  agreeing  duplicates. 


TABLE  IV 

Effect  of 

Sol 

uble  Humus 

No. 

1 

Soluble 

humus  added 

Grs. 

0.2 

Nitrate  N 
added 
Mgs. 

-     100 

Nitrate  N 
recovered 

Mgs. 

95.62 

2 

0.2 

50 

48.86 

3 

0.2 

25 

24.36 

Effect  of  Dextrose 


No. 

Dextrose 
added 
Grs. 

Nitrate  N 
added 
Mgs. 

Nitrate  N 

recovered 

Mgs. 

1 

1.0 

100 

96.60 

2 

1.0 

50 

48.44 

3 

1.0 

25 

24.50 

60  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

These  results  show  a  slight  loss  where  large  amounts  of  nitrate 
were  present,  but  the  same  also  holds  true  above,  as  an  exami- 
nation of  the  foregoing  tables  will  indicate.  Therefore  the  losses, 
which  are  also  in  part  explained  above,  are  not  to  be  attributed 
to  the  presence  of  excessive  amounts  of  soluble  organic  materials. 
Besides,  the  losses  brought  about  for  the  reasons  indicated  are 
of  no  serious  practical  import,  since  most  soils  seldom  contain 
more  than  from  15  to  20  milligrams  of  nitrogen  per  100  grams 
of  soil. 

The  Time  Required  for  Complete  Reduction  at  a 
Temperature  of  20°-22°  C. 

That  the  time  required  for  complete  reduction  should  be 
accurately  ascertained  at  a  given  temperature  is  obvious.  The 
results  given  below  are  averages  of  analyses  carried  on  simul- 
taneously. 

TABLE  V 

The  Effect  of  Time  on  the  Process  of  Eeduction  at  Constant 
Temperature  (2o°'22°  C.) 

12  15  24 

97.44  98.42  95.20 
48.86  49.42  49.42 
24.50     24.50     24.50 

In  all  of  these  samples  0.2  per  cent  of  NaCl  was  present.  A 
study  of  this  data  shows  11  to  15  hours  to  be  the  optimum  time 
limits  for  reduction  at  20°-22°  C.  A  longer  time  results  in  a 
slight  loss  of  ammonia  from  the  solutions  containing  the  larger 
amounts  of  nitrogen,  while  less  than  11  hours  is  too  short  to  effect 
complete  reduction. 

The  Effect  of  Temperature  on  the  Time  Necessary  for 

Reduction 

Temperature  is  of  vital  importance,  both  in  the  accuracy  of 
the  determination  and  also  on  the  time  necessary  for  complete 
reaction.    This  is  borne  out  by  the  following  table. 


Hours          1 

Nitrate  N 
added 

2V2 

4 

6 

8                9              10 

Nitrate  N  recovered 

Mgs. 

n 

100      19.04 
50     19.18 
25     14.35 

33.88 
22.05 
17.71 

46.20 
31.36 

55.02 
36.82 
19.00 

89.74     93.10     96.53 
46.76     47.18     49.14 
20.79     23.94     23.94 

97.90 
49.10 
24.50 

1913]  Burgess:   Reduction  Methods  for  Soil  Nitrates  61 

TABLE  VI 

Time  of  Reduction  12  Hours.    Temperature  Constant  at  8°  to  10°  C. 

NaCl  Nitrate  N  Nitrate  N 

added  added  recovered 

Grams  Mgs.  Mgs. 

.2  100  47.53 

.2  50  28.42 

.2  25  22.96 

Time  of  Reduction  6  Hours.    Temperature  Constant  at  45°  C. 

NaCl  Nitrate  N  Nitrate  N 

added  added  recovered 

Grams  Mgs.  Mgs. 

.2  100  80.93 

.2  50         43.26 

.2  25         22.12 

We  thus  see  that  at  a  temperature  of  45°  C.  the  reduction 
was  more  nearly  complete  in  6  hours  than  it  was  at  the  end  of 
12  hours  where  the  temperature  was  maintained  at  10°  C.  By 
means  of  a  series  of  experiments  similar  to  those  shown  in  Table 
IV  it  is  readily  possible  to  find  the  optimum  length  of  time  for 
complete  reaction  at  any  given  temperature. 

Amount  NaOH  Solution  Required 

By  another  series  of  experiments,  in  which  the  traps  above 
mentioned  were  used  and  different  amounts  (varying*  from  1  to 
5c.c.)  of  the  50  per  cent  NaOH  solution  added,  we  found  that 
2  c.c.  of  the  latter  gave  the  best  results.  One  c.c.  evidently  did  not 
generate  hydrogen  in  sufficient  quantities  for  complete  reduction 
(especially  in  the  presence  of  the  larger  amounts  of  nitrate), 
while  the  4  and  5  c.c.  portions  made  the  solutions  so  alkaline  as 
to  effect  the  escape  of  considerable  amounts  of  ammonia. 

General  Remarks 

The  results  obtained  in  the  experiments  described  above  are 
considered  a  valuable  contribution  to  our  methods  of  chemical 
and  biochemical  soil  analysis,  because  they  establish,  on  a  prac- 
tical basis,  a  method  for  the  determination  of  nitrates  in  soils 
which  yields  accurate  results,  regardless  of  the  condition  of  the 
soil  which  is  examined.  While  for  rapid  determinations  the 
reduction  method  is  not  found  as  serviceable  as  the  phenoldisul- 
phonic  acid  method,  it  replaces  the  latter  for  work  on  "alkali" 
bearing  soils  where  it  alone  can  be  safely  employed. 


62  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

Moreover,  the  writer's  results  are  also  proof  of  the  fact,  as 
the  data  in  Table  I  indicate,  that  even  in  the  absence  of  salts  the 
reduction  method  is  superior  to  the  colorimetric  method  where 
large  quantities  of  nitrates  are  concerned. 

In  the  case  of  soils  containing  large  amounts  of  organic  matter 
the  reduction  method  is  again  found  superior,  since  it  is  fre- 
quently found  impossible  to  remove  the  color  of  a  soil  extract 
from  such  soils  without  a  loss  of  nitrates. 

So  far  as  my  investigations  have  gone  on  the  subject,  and  I 
have  tried  to  take  into  consideration  the  several  factors  involved, 
no  uncontrollable  factor  has  been  found  to  militate  against  the 
successful  use  of  the  aluminum  reduction  method.  Great  accu- 
racy can  be  obtained  by  taking  the  precautions  above  described, 
and  reasonable  accuracy  may  be  assured  in  ordinary  work  with- 
out the  use  of  such  extraordinary  measures.  The  method  is 
strongly  commended  to  the  attention  of  soil  investigators  in  all 
phases  of  the  work  and  it  is  to  be  hoped  that  it  may  serve  to 
render  simple  and  free  from  annoyance  the  determination  of 
nitrates  in  soils  under  circumstances  which  until  now  have  pre- 
sented many  difficulties. 

CONCLUSIONS 

1.  The  aluminum  reduction  method  for  the  determination  of 
nitrates  in  soils  yields  the  most  accurate  results  of  all  methods 
now  commonly  in  vogue. 

2.  "Alkali"  salts  do  not  in  any  way  interfere  with  the  success- 
ful operation  of  the  method. 

3.  The  presence  of  extraordinarily  large  amounts  of  soluble 
organic  materials  (soluble  humus  and  dextrose)  have  little  effect 
on  the  method. 

4.  A  temperature  of  20°  C.  for  from  11  to  15  hours  has  been 
found  the  optimum  for  the  reduction  of  large  quantities  of 
nitrates. 

5.  The  proper  amount  of  NaOH  to  be  employed  in  the  reduc- 
tion was  found  to  be  2  c.c.  of  a  50  per  cent  solution,  with  an 
aluminum  strip  weighing  approximately  one  gram. 

My  thanks  are  due  Prof.  C.  B.  Lipman  for  helpful  sugges- 
tions and  critical  reading  of  the  manuscript. 


